The multi-pulling process is a known technique for producing silicon single crystals using the CZ method (See, Patent Document 1, for example). In the multi-pulling process, after a silicon single crystal has been pulled up, an additional amount of the silicon material is fed to the same quartz crucible to replenish the silicon and melted. From the resultant silicon melt, a subsequent silicon single crystal is pulled up. Such feeding and pulling steps are repeated to produce multiple silicon single crystals from the single quartz crucible. According to the multi-pulling process, it is possible to reduce the cost of the quartz crucible per each silicon single crystal produced. In addition, the process can reduce the frequency of disassembling the chamber to replace the quartz crucible, thereby improving the operation efficiency.
During the silicon single crystal production using the CZ method, a silicon material such as polycrystalline silicon is charged into a quartz crucible and melted by heating in a chamber. Subsequently, a seed crystal mounted on the lower end of a pulling rod is descended from above the quartz crucible and dipped in the silicon melt. The seed crystal is then slowly lifted upward while rotating the seed crystal and the quartz crucible in predetermined directions, thus resulting in the growth of a single crystal underneath the seed crystal.
The chamber is maintained under reduced pressure during the growth of the silicon single crystal. While the silicon melt is supersaturated with oxygen dissolved from the quartz crucible, the gas flow rate is slowed when the pressure inside the chamber (furnace pressure) is high. As a result, a significant amount of evaporated SiO is deposited inside the chamber, which may fall in the silicon melt as SiO powder and cause dislocations. However, the occurrence of dislocation can be minimized by decreasing the furnace pressure so that the generated gas can be effectively discharged from the furnace.
In order to solve the problem of dislocations in single crystals caused by the evaporation of SiO and by the formation of pinholes, Patent Document 2 describes a method for controlling a furnace pressure in which the furnace pressure during the melting step is maintained at from 65 to 400 mbar and the furnace pressure during the pulling step is maintained lower than that and 95 mbar or below. Also, Patent Document 3 describes a method for controlling a furnace pressure in which the furnace pressure during the first half of the melting step is maintained at a higher pressure (60 to 400 mbar) and the furnace pressure during the second half of the melting step is maintained at a lower pressure (60 mbar or below). Further, Patent Document 4 describes a method for preventing pinhole defects in a silicon single crystal in which the furnace pressure during melting of the material is maintained at a low pressure below 60 hPa to facilitate the volatilization of the gas dissolved in the silicon melt.